1. Field of the Invention
This invention relates generally to an interbody device for restoring the disc space height between two vertebrae during spinal fusion surgery and, more particularly, to an interbody device for restoring the disc space height between two vertebrae during minimally invasive spinal fusion surgery, where the device also allows for an effective distribution of bone graft material in the disc space.
2. Discussion of the Related Art
The human spine includes a series of vertebrae interconnected by connective tissue referred to as discs that act as a cushion between the vertebrae. The discs allow for movement of the vertebrae so that the back can bend and rotate.
Spinal fusion is a surgical procedure that fuses two or more vertebrae together using bone grafts and/or other devices. Spinal fusion is a commonly performed procedure for the treatment of chronic neck and back pain refractory to non-operative treatments. Spinal fusion is used to stabilize or eliminate motion of vertebrae segments that may be unstable, i.e., move in an abnormal way, that may lead to pain and discomfort. Spinal fusion is typically performed to treat injuries to the vertebrae, degeneration of the spinal discs, abnormal spinal curvature and a weak or unstable spine.
Spinal fusion generally requires a graft material, usually bone material, to fuse the vertebrae together. The bone graft material can be placed over the spine to fuse adjacent vertebrae together. Alternatively, a cage is positioned between the vertebrae being fused, and is filed with the bone graft material. The cage includes holes that allow the vertebrae and the graft material to grow together to provide the fusion. The cage supports the weight of adjacent vertebrae while the fusion is occurring through the holes in the cage.
Typically the bone graft material is autogenous bone material taken from the patient, or allograft bone material harvested from a cadaver. Synthetic bone material can also be used as the graft material. Generally, the patient's own bone material offers the best fusion material and is the current “gold standard.” Known bone fusion materials include an iliac crest harvest from the patient, bone graft extenders, such as hydroxyapetite and demineralized bone matrix, and bone morphogenic protein.
In an attempt to preserve normal anatomical structures during spinal surgery, minimally invasive surgical procedures have been devised. One such procedure involves the use of a series of muscle dilators that separate the muscle fibers of the spine to create a pathway to the spine. A Kirschner (K-wire) is initially introduced through a small incision and directed towards the spinal pathology. The position of the K-wire is visualized by a fluoroscopic imaging system to identify its location. An initial narrow diameter muscle dilator is passed over the K-wire, and the K-wire is removed and subsequent larger muscle dilators are continually passed. When the opening is large enough, an access tube or retractor is positioned around the last muscle dilator through which the surgery is performed. The inner sequential muscle dilators are then removed allowing the surgeon to operate through the tubular retractor. The retractors come in a variety of lengths and diameters for different patients and procedures.
As mentioned above, a cage is typically positioned in the interbody region between the vertebrae after the disc has been removed. These cages typically have a box like design. The cage is forced into the interbody region through the surgical area where the bone and disc have been removed. The cage is filled with the bone graft material that subsequently fuses the vertebrae together. However, known cage designs are limited in that they only allow for partial filling of the interbody space where the graft material is maintained within the cage, thus only allowing partial fusion between the vertebrae. Further, the known bone graft cages are difficult to place because of their square or cylindrical shape, and put the nerve roots at risk during the procedure, sometimes resulting in injury. Also, the known cages do not allow the collapsed disc space height to be fully restored in that they cannot distract the open disc space once they are in place. Further, the known cage designs require that the bone graft material be placed in the cage prior to it being inserted into the interbody region, which limits the amount of bone material placed in the disc space and subsequent fusion surface. Also, once the cages are placed, they are difficult to remove and reposition.